# How does resistance, voltage and current work in a circuit with a small load

If I intend to run an LED with a 5V source voltage and an LED forward voltage of 2V at 20ma it is my understanding that a 150 ohm resistor is suited.

I get this from using the formula V = IR rearranged to R = V/I. R = 3/0.02 A 3V voltage difference at 20ma.

This works in practice as well, the LED is well lit and I get the forward voltage and current I expect when measured.

How exactly are these concepts related? It is my understanding that at at V voltage with R resistance, a maximum of I current will be allowed through. Is my understanding of these concepts over simplified or valid?

• "At V voltage" is inaccurate. – Ignacio Vazquez-Abrams Jun 12 '18 at 23:11
• Could you elaborate? Is my concept of voltage misguided? – Zano Jun 12 '18 at 23:13
• Note that if the LED has a maximum rating of 20 mA and you run it at 20 mA continuously, it will not have a long life. Either PWM it or run it at 50% to 75% of its max rating for a long life. – Sparky256 Jun 12 '18 at 23:14
• From what you have written here your understanding appears reasonable and valid. I could be missing something but I don't see any logic flaws. – Redja Jun 12 '18 at 23:20
• The "a maximum of" part is mistaken. Current, voltage, and resistance are precisely related - or one of the measurements is wrong or inapplicable. For example, while it works for the resistor, you can't really express the behavior of an LED as a simple resistance. Given that the forward voltage on an LED is not ultimately fixed, there are still a number of unknowns. – Chris Stratton Jun 12 '18 at 23:23